Coding of picture order count in video bitstreams

ABSTRACT

Methods, systems and devices for coding the picture order count in video bitstream processing. An example method of video processing includes performing a conversion between a video comprising one or more pictures and a bitstream of the video, wherein the bitstream conforms to a format rule, wherein the format rule specifies that a derivation of a picture associated with a first flag and in a decoding process for a picture order count is based on a second flag, wherein the picture associated with the first flag is a previous picture in a decoding order that has (i) a first identifier that is the same as a slice or picture header referring to a reference picture list syntax structure, (ii) a second identifier and the second flag being equal to zero, and (iii) a picture type different from a random access skipped leading picture and a random access decodable leading picture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/US2021/033780, filed on May 21, 2021 which claims the priorityto and benefits of U.S. Provisional Patent Application No. 63/029,321filed on May 22, 2020. All the aforementioned patent applications arehereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to image and video coding and decoding.

BACKGROUND

Digital video accounts for the largest bandwidth use on the internet andother digital communication networks. As the number of connected userdevices capable of receiving and displaying video increases, it isexpected that the bandwidth demand for digital video usage will continueto grow.

SUMMARY

The present disclosure discloses embodiments that can be used by videoencoders and decoders to perform video encoding or decoding.

In an example aspect, a video processing method is disclosed. The methodincludes performing a conversion between a video comprising one or morepictures and a bitstream of the video, wherein the bitstream conforms toa format rule, wherein the format rule specifies that a picture timing(PT) supplemental enhancement information (SEI) message, when includedin the bitstream, is access unit (AU) specific, and wherein each pictureof the one or more pictures that is a random access skipped leading(RASL) picture includes only a RASL network abstraction layer unit type(NUT).

In another example aspect, another video processing method is disclosed.The method includes performing a conversion between a video comprisingone or more pictures and a bitstream of the video, wherein the bitstreamconforms to a format rule, wherein the format rule permits a use of arandom access decodable leading (RADL) subpicture in a random accessskipped leading (RASL) picture as a reference subpicture for predictinga collocated RADL picture in a RADL picture associated with a same cleanrandom access (CRA) picture as the RASL picture.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a derivation of a picture associated with a first flagand in a decoding process for a picture order count is based on a secondflag, wherein the picture associated with the first flag is a previouspicture in a decoding order that has (i) a first identifier same as aslice or picture header referring to a reference picture list syntaxstructure, (ii) a second identifier and the second flag being equal tozero, and (iii) a picture type different from a random access skippedleading (RASL) picture and a random access decodable leading (RADL)picture, wherein the first flag indicates whether a third flag ispresent in the bitstream, wherein the second flag indicates whether acurrent picture is used as a reference picture, and wherein the thirdflag is used to determine a value of one or more most significant bitsof a picture order count value of a long-term reference picture.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a variable, which is used to determine a timing of aremoval of a decoding unit (DU) or decoding the DU, is access unit (AU)specific and derived based on a flag that indicates whether a currentpicture is allowed to be used as a reference picture.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a buffering period supplemental enhancement information(SEI) message and a picture timing SEI message, when included in thebitstream, are access unit (AU) specific, wherein a first variableassociated with the buffering period SEI message and a second variableassociated with the buffering period SEI message and the picture timingSEI message are derived based on a flag that indicates whether a currentpicture is allowed to be used as a reference picture, wherein the firstvariable is indicative of an access unit comprising (i) an identifierthat is equal to zero and (ii) a picture that is not a random accessskipped leading (RASL) picture or a random access decodable leading(RADL) picture and for which the flag is equal to zero, and wherein thesecond variable is indicative of a current AU not being a first AU in adecoding order and a previous AU in the decoding order comprising (i)the identifier that is equal to zero and (ii) a picture that is not arandom access skipped leading (RASL) picture or a random accessdecodable leading (RADL) picture and for which the flag is equal tozero.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a derivation of a first variable and a second variableassociated with a first picture and a second picture is based on a flag,wherein the first picture is a current picture and the second picture isa previous picture in a decoding order that (i) comprises a firstidentifier that is equal to zero, (ii) comprises a flag that is equal tozero, and (iii) is not a random access skipped leading (RASL) picture ora random access decodable leading (RADL) picture, and wherein the firstvariable and the second variable are a maximum value and a minimumvalue, respectively, of a picture order count of each of the followingpictures with a second identifier that is equal to that of the firstpicture (i) the first picture, (ii) the second picture, (iii) one ormore short-term reference pictures referred to by all entries inreference picture lists of the first picture, and (iv) each picture thathas been output with a coded picture buffer (CPB) removal time less thanthe CPB removal time of the first picture and a decoded picture buffer(DPB) output time greater than or equal to the CPB removal time of thefirst picture.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a flag and a syntax element, when included in thebitstream, are access unit (AU) specific, wherein the flag indicates, inresponse to a current AU not being a first AU in the bitstream in adecoding order, whether a nominal coded picture buffer (CPB) removaltime of the current AU is determined relative to (a) a nominal CPBremoval time of a previous AU associated with a buffering periodsupplemental enhancement information (SEI) message or (b) a nominal CPBremoval time of the current AU, and wherein the syntax elementspecifies, in response to a current AU not being a first AU in thebitstream in a decoding order, a CPB removal delay increment valuerelative to the nominal CPB removal time of the current AU.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a plurality of variables and a picture timingsupplemental enhancement information (SEI) message, when included in thebitstream, are access unit (AU) specific, wherein the picture timing SEImessage comprises a plurality of syntax elements, wherein a firstvariable of the plurality of variables indicates whether a current AU isassociated with a buffering period SEI message, wherein a secondvariable and a third variable of the plurality of variables areassociated with an indication of whether the current AU is an AU thatinitializes a hypothetical reference decoder (HRD), wherein a firstsyntax element of the plurality of syntax elements specifies a number ofclock ticks to wait after a removal of an AU from a coded picture buffer(CPB) before one or more decoded pictures of the AU are output from thedecoded picture buffer (DPB), wherein a second syntax element of theplurality of syntax elements specifies a number of sub clock ticks towait after a removal of a last decoding unit (DU) in an AU from the CPBbefore the one or more decoded pictures of the AU are output from theDPB, and wherein a third syntax element of the plurality of syntaxelements specifies a number of elemental picture period intervals thatone or more decoded pictures of the current AU occupy for a displaymodel.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a syntax element associated with a decoding picturebuffer (DPB), when included in the bitstream, is access unit (AU)specific, and wherein the syntax element specifies a number of sub clockticks to wait after removal of a last decoding unit (DU) in an AU fromthe coded picture buffer (CPB) before one or more decoded pictures ofthe AU are output from the DPB.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a flag, when included in the bitstream, is access unit(AU) specific, wherein the value of the flag is based on whether anassociated AU is an intra random access points (IRAP) AU or a gradualdecoding refresh (GDR) AU, and wherein the value of the flag specifies(i) whether a syntax element is present in a buffering periodsupplemental enhancement information (SEI) message and (ii) whetheralternative timing information is present in a picture timing SEImessage in a current buffering period.

In yet another example aspect, another video processing method isdisclosed. The method includes performing a conversion between a videocomprising one or more pictures and a bitstream of the video, whereinthe bitstream conforms to a format rule, wherein the format rulespecifies that a value of a first syntax element is based on a flag thatindicates whether a temporal distance between output times ofconsecutive pictures of a hypothetical reference decoder (HRD) isconstrained and a variable that identifies a highest temporal sublayerto be decoded, and wherein the first syntax element specifies a numberof elemental picture period intervals that one or more decoded picturesof the current AU occupy for a display model.

In yet another example aspect, a video encoder apparatus is disclosed.The video encoder comprises a processor configured to implementabove-described methods.

In yet another example aspect, a video decoder apparatus is disclosed.The video decoder comprises a processor configured to implementabove-described methods.

In yet another example aspect, a computer readable medium having codestored thereon is disclosed. The code embodies one of the methodsdescribed herein in the form of processor-executable code.

These, and other, features are described throughout the presentdisclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an example video processing system inwhich various embodiments disclosed herein may be implemented.

FIG. 2 is a block diagram of an example hardware platform used for videoprocessing.

FIG. 3 is a block diagram that illustrates an example video codingsystem that can implement some embodiments of the present disclosure.

FIG. 4 is a block diagram that illustrates an example of an encoder thatcan implement some embodiments of the present disclosure.

FIG. 5 is a block diagram that illustrates an example of a decoder thatcan implement some embodiments of the present disclosure.

FIGS. 6-16 show flowcharts for example methods of video processing.

DETAILED DESCRIPTION

Section headings are used in the present disclosure for ease ofunderstanding and do not limit the applicability of embodimentsdisclosed in each section only to that section. Furthermore, H.266terminology is used in some description only for ease of understandingand not for limiting scope of the disclosed embodiments. As such, theembodiments described herein are applicable to other video codecprotocols and designs also.

1. INTRODUCTION

This disclosure is related to video coding technologies. Specifically,it is about handling of discardable pictures/access units (AUs) andsemantics of hypothetical reference decoder (HRD)-related SEI messagesin video coding. Examples of discardable pictures that can be discardedin certain scenarios include random access skipped leading (RASL)pictures, random access decodable leading (RADL) pictures, and pictureswith ph_non_ref_pic_flag equal to 1. HRD related SEI messages includebuffering period (BP), picture timing (PT), and decoding unitinformation (DUI) SEI messages. The ideas may be applied individually orin various combination, to any video coding standard or non-standardvideo codec that supports multi-layer video coding, e.g., thebeing-developed Versatile Video Coding (VVC).

2. ABBREVIATIONS

APS Adaptation Parameter Set

AU Access Unit

AUD Access Unit Delimiter

AVC Advanced Video Coding

CLVS Coded Layer Video Sequence

CPB Coded Picture Buffer

CRA Clean Random Access

CTU Coding Tree Unit

CVS Coded Video Sequence

DCI Decoding Capability Information

DPB Decoded Picture Buffer

EOB End Of Bitstream

EOS End Of Sequence

GDR Gradual Decoding Refresh

HEVC High Efficiency Video Coding

HRD Hypothetical Reference Decoder

IDR Instantaneous Decoding Refresh

ILP Inter-Layer Prediction

ILRP Inter-Layer Reference Picture

IRAP Intra Random Access Points

JEM Joint Exploration Model

LTRP Long-Term Reference Picture

MCTS Motion-Constrained Tile Sets

NAL Network Abstraction Layer

OLS Output Layer Set

PH Picture Header

PPS Picture Parameter Set

PTL Profile, Tier and Level

PU Picture Unit

RAP Random Access Point

RADL Random Access Decodable Leading Picture

RASL Random Access Skipped Leading Picture

RBSP Raw Byte Sequence Payload

SEI Supplemental Enhancement Information

SPS Sequence Parameter Set

STRP Short-Term Reference Picture

SVC Scalable Video Coding

VCL Video Coding Layer

VPS Video Parameter Set

VTM VVC Test Model

VUI Video Usability Information

VVC Versatile Video Coding

3. INITIAL DISCUSSION

Video coding standards have evolved primarily through the development ofthe well-known International Telecommunication Union (ITU)Telecommunication Standardization Sector (ITU-T) and InternationalOrganization for Standardization (ISO)/International ElectrotechnicalCommission (IEC) standards. The ITU-T produced H.261 and H.263, ISO/IECproduced Moving Picture Experts Group (MPEG)-1 and MPEG-4 Visual, andthe two organizations jointly produced the H.262/MPEG-2 Video andH.264/MPEG-4 Advanced Video Coding (AVC) and H.265/HEVC standards. SinceH.262, the video coding standards are based on the hybrid video codingstructure wherein temporal prediction plus transform coding areutilized. To explore the future video coding technologies beyond HEVC,the Joint Video Exploration Team (JVET) was founded by Video CodingExperts Group (VCEG) and MPEG jointly in 2015. Since then, many newmethods have been adopted by JVET and put into the reference softwarenamed Joint Exploration Model (JEM). The JVET meeting is concurrentlyheld once every quarter, and the new coding standard is targeting a 50%bitrate reduction as compared to HEVC. The new video coding standard wasofficially named as Versatile Video Coding (VVC) in the April 2018 JVETmeeting, and the first version of VVC test model (VTM) was released atthat time. As there are continuous effort contributing to VVCstandardization, new coding techniques are being adopted to the VVCstandard in every JVET meeting. The VVC working draft and test model VTMare then updated after every meeting. The VVC project is now aiming fortechnical completion (FDIS) at the July 2020 meeting.

3.1. Reference Picture Management and Reference Picture Lists (RPLs)

Reference picture management is a core functionality that is necessaryfor any video coding scheme that uses inter prediction. It manages thestorage and removal of reference pictures into and from a decodedpicture buffer (DPB) and puts reference pictures in their proper orderin the RPLs.

The reference picture management of HEVC, including reference picturemarking and removal from the decoded picture buffer (DPB) as well asreference picture list construction (RPLC), differs from that of AVC.Instead of the reference picture marking mechanism based on a slidingwindow plus adaptive memory management control operation (MMCO) in AVC,HEVC specifies a reference picture management and marking mechanismbased on so-called reference picture set (RPS), and the RPLC isconsequently based on the RPS mechanism. An RPS consists of a set ofreference pictures associated with a picture, consisting of allreference pictures that are prior to the associated picture in decodingorder, that may be used for inter prediction of the associated pictureor any picture following the associated picture in decoding order. Thereference picture set consists of five lists of reference pictures. Thefirst three lists contain all reference pictures that may be used ininter prediction of the current picture and that may be used in interprediction of one or more of the pictures following the current picturein decoding order. The other two lists consist of all reference picturesthat are not used in inter prediction of the current picture but may beused in inter prediction of one or more of the pictures following thecurrent picture in decoding order. RPS provides an “intra-coded”signalling of the DPB status, instead of an “inter-coded”signalling asin AVC, mainly for improved error resilience. The RPLC process in HEVCis based on the RPS, by signalling an index to an RPS subset for eachreference index; this process is simpler than the RPLC process in AVC.

Reference picture management in VVC is more similar to HEVC than AVC,but is somewhat simpler and more robust. As in those standards, twoRPLs, list 0 and list 1, are derived, but they are not based on thereference picture set concept used in HEVC or the automatic slidingwindow process used in AVC; instead they are signalled more directly.Reference pictures are listed for the RPLs as either active and inactiveentries, and only the active entries may be used as reference indices ininter prediction of CTUs of the current picture. Inactive entriesindicate other pictures to be held in the DPB for referencing by otherpictures that arrive later in the bitstream.

3.2. Random Access and its Supports in HEVC and VVC

Random access refers to starting access and decoding of a bitstream froma picture that is not the first picture of the bitstream in decodingorder. To support tuning in and channel switching in broadcast/multicastand multiparty video conferencing, seeking in local playback andstreaming, as well as stream adaptation in streaming, the bitstreamneeds to include frequent random access points, which are typicallyintra coded pictures but may also be inter-coded pictures (e.g., in thecase of gradual decoding refresh).

HEVC includes signalling of intra random access points (IRAP) picturesin the NAL unit header, through NAL unit types. Three types of IRAPpictures are supported, namely instantaneous decoder refresh (IDR),clean random access (CRA), and broken link access (BLA) pictures. IDRpictures are constraining the inter-picture prediction structure to notreference any picture before the current group-of-pictures (GOP),conventionally referred to as closed-GOP random access points. CRApictures are less restrictive by allowing certain pictures to referencepictures before the current GOP, all of which are discarded in case of arandom access. CRA pictures are conventionally referred to as open-GOPrandom access points. BLA pictures usually originate from splicing oftwo bitstreams or part thereof at a CRA picture, e.g., during streamswitching. To enable better systems usage of IRAP pictures, altogethersix different NAL units are defined to signal the properties of the IRAPpictures, which can be used to better match the stream access pointtypes as defined in the ISO base media file format (ISOBMFF), which areutilized for random access support in dynamic adaptive streaming overHTTP (DASH).

VVC supports three types of IRAP pictures, two types of IDR pictures(one type with or the other type without associated RADL pictures) andone type of CRA picture. These are basically the same as in HEVC. TheBLA picture types in HEVC are not included in VVC, mainly due to tworeasons: i) The basic functionality of BLA pictures can be realized byCRA pictures plus the end of sequence NAL unit, the presence of whichindicates that the subsequent picture starts a new CVS in a single-layerbitstream. ii) There was a desire in specifying less NAL unit types thanHEVC during the development of VVC, as indicated by the use of fiveinstead of six bits for the NAL unit type field in the NAL unit header.

Another key difference in random access support between VVC and HEVC isthe support of GDR in a more normative manner in VVC. In GDR, thedecoding of a bitstream can start from an inter-coded picture andalthough at the beginning not the entire picture region can be correctlydecoded but after a number of pictures the entire picture region wouldbe correct. AVC and HEVC also support GDR, using the recovery point SEImessage for signalling of GDR random access points and the recoverypoints. In VVC, a new NAL unit type is specified for indication of GDRpictures and the recovery point is signalled in the picture headersyntax structure. A CVS and a bitstream are allowed to start with a GDRpicture. This means that it is allowed for an entire bitstream tocontain only inter-coded pictures without a single intra-coded picture.The main benefit of specifying GDR support this way is to provide aconforming behavior for GDR. GDR enables encoders to smooth the bit rateof a bitstream by distributing intra-coded slices or blocks in multiplepictures as opposed intra coding entire pictures, thus allowingsignificant end-to-end delay reduction, which is considered moreimportant nowadays than before as ultralow delay applications likewireless display, online gaming, drone based applications become morepopular.

Another GDR related feature in VVC is the virtual boundary signalling.The boundary between the refreshed region (i.e., the correctly decodedregion) and the unrefreshed region at a picture between a GDR pictureand its recovery point can be signalled as a virtual boundary, and whensignalled, in-loop filtering across the boundary would not be applied,thus a decoding mismatch for some samples at or near the boundary wouldnot occur. This can be useful when the application determines to displaythe correctly decoded regions during the GDR process.

IRAP pictures and GDR pictures can be collectively referred to as randomaccess point (RAP) pictures.

3.3. Picture Resolution Change within a Sequence

In AVC and HEVC, the spatial resolution of pictures cannot change unlessa new sequence using a new SPS starts, with an intra random accesspoints (IRAP) picture. VVC enables picture resolution change within asequence at a position without encoding an IRAP picture, which is alwaysintra-coded. This feature is sometimes referred to as reference pictureresampling (RPR), as the feature needs resampling of a reference pictureused for inter prediction when that reference picture has a differentresolution than the current picture being decoded.

The scaling ratio is restricted to be greater than or equal to 1/2 (2times downsampling from the reference picture to the current picture),and less than or equal to 8 (8 times upsampling). Three sets ofresampling filters with different frequency cutoffs are specified tohandle various scaling ratios between a reference picture and thecurrent picture. The three sets of resampling filters are appliedrespectively for the scaling ratio ranging from 1/2 to 1/1.75, from1/1.75 to 1/1.25, and from 1/1.25 to 8. Each set of resampling filtershas 16 phases for luma and 32 phases for chroma which is same to thecase of motion compensation interpolation filters. Actually, the normalMC interpolation process is a special case of the resampling processwith scaling ratio ranging from 1/1.25 to 8. The horizontal and verticalscaling ratios are derived based on picture width and height, and theleft, right, top and bottom scaling offsets specified for the referencepicture and the current picture.

Other aspects of the VVC design for support of this feature that aredifferent from HEVC include: i) the picture resolution and thecorresponding conformance window are signalled in the PPS instead of inthe SPS, while in the SPS the maximum picture resolution is signalled;and ii) for a single-layer bitstream, each picture store (a slot in theDPB for storage of one decoded picture) occupies the buffer size asrequired for storing a decoded picture having the maximum pictureresolution.

3.4. Scalable Video Coding (SVC) in General and in VVC

Scalable video coding (SVC, sometimes also just referred to asscalability in video coding) refers to video coding in which a baselayer (BL), sometimes referred to as a reference layer (RL), and one ormore scalable enhancement layers (ELs) are used. In SVC, the base layercan carry video data with a base level of quality. The one or moreenhancement layers can carry additional video data to support, forexample, higher spatial, temporal, and/or signal-to-noise (SNR) levels.Enhancement layers may be defined relative to a previously encodedlayer. For example, a bottom layer may serve as a BL, while a top layermay serve as an EL. Middle layers may serve as either ELs or RLs, orboth. For example, a middle layer (e.g., a layer that is neither thelowest layer nor the highest layer) may be an EL for the layers belowthe middle layer, such as the base layer or any intervening enhancementlayers, and at the same time serve as a RL for one or more enhancementlayers above the middle layer. Similarly, in the multiview orthree-dimensional (3D) extension of the HEVC standard, there may bemultiple views, and information of one view may be utilized to code(e.g., encode or decode) the information of another view (e.g., motionestimation, motion vector prediction and/or other redundancies).

In SVC, the parameters used by the encoder or the decoder are groupedinto parameter sets based on the coding level (e.g., video-level,sequence-level, picture-level, slice level, etc.) in which they may beutilized. For example, parameters that may be utilized by one or morecoded video sequences of different layers in the bitstream may beincluded in a video parameter set (VPS), and parameters that areutilized by one or more pictures in a coded video sequence may beincluded in a sequence parameter set (SPS). Similarly, parameters thatare utilized by one or more slices in a picture may be included in apicture parameter set (PPS), and other parameters that are specific to asingle slice may be included in a slice header. Similarly, theindication of which parameter set(s) a particular layer is using at agiven time may be provided at various coding levels.

Thanks to the support of reference picture resampling (RPR) in VVC,support of a bitstream containing multiple layers, e.g., two layers withstandard definition (SD) and high definition (HD) resolutions in VVC canbe designed without the need any additional signal-processing-levelcoding tool, as upsampling needed for spatial scalability support canjust use the RPR upsampling filter. Nevertheless, high-level syntaxchanges (compared to not supporting scalability) are needed forscalability support. Scalability support is specified in VVC version 1.Different from the scalability supports in any earlier video codingstandards, including in extensions of AVC and HEVC, the design of VVCscalability has been made friendly to single-layer decoder designs asmuch as possible. The decoding capability for multi-layer bitstreams arespecified in a manner as if there were only a single layer in thebitstream. For example, the decoding capability, such as DPB size, isspecified in a manner that is independent of the number of layers in thebitstream to be decoded. Basically, a decoder designed for single-layerbitstreams does not need much change to be able to decode multi-layerbitstreams. Compared to the designs of multi-layer extensions of AVC andHEVC, the hypertext transfer protocol live streaming (HLS) aspects havebeen significantly simplified at the sacrifice of some flexibilities.For example, an IRAP AU is required to contain a picture for each of thelayers present in the CVS.

3.5. Parameter Sets

AVC, HEVC, and VVC specify parameter sets. The types of parameter setsinclude SPS, PPS, APS, and VPS. SPS and PPS are supported in all of AVC,HEVC, and VVC. VPS was introduced since HEVC and is included in bothHEVC and VVC. APS was not included in AVC or HEVC but is included in thelatest VVC draft text.

SPS was designed to carry sequence-level header information, and PPS wasdesigned to carry infrequently changing picture-level headerinformation. With SPS and PPS, infrequently changing information neednot to be repeated for each sequence or picture, hence redundantsignalling of this information can be avoided. Furthermore, the use ofSPS and PPS enables out-of-band transmission of the important headerinformation, thus not only avoiding the need for redundant transmissionsbut also improving error resilience.

VPS was introduced for carrying sequence-level header information thatis common for all layers in multi-layer bitstreams.

APS was introduced for carrying such picture-level or slice-levelinformation that needs quite some bits to code, can be shared bymultiple pictures, and in a sequence there can be quite many differentvariations.

4. TECHNICAL PROBLEMS SOLVED BY THE DISCLOSED TECHNICAL SOLUTIONS

The existing designs for handling of discardable pictures and AUs in thelatest VVC text (in JVET-R2001-vA/v10) have the following problems:

-   -   1) In clause 3 (Definitions), as part of the definition of RASL        picture, there is an issue with the sentence “RASL pictures are        not used as reference pictures for the decoding process of        non-RASL pictures,” because that is no longer true in some        scenarios, thus it would cause confusion and interoperability        problems.    -   2) In clause D.4.2 (Picture timing SEI message semantics), the        constraint on pt_cpb_alt_timing_info_present_flag being equal to        0 is specified in a picture-specific manner. However, as HRD        operations are OLS-based, the semantics are technically        incorrect and can cause interoperability problems.    -   3) The derivation of prevTid0Pic in the semantics of        delta_poc_msb_cycle_present_flag[i][j] and in the decoding        process for picture order count (POC) does not take into account        the value of ph_non_ref_pic_flag. This would cause problems when        pictures with ph_non_ref_pic_flag equal to 1 are discarded, as        then the derived POC values for pictures and/or the signalled        reference pictures can be wrong, and unexpected incorrect        decoding behaviors, including decoder crash, can occur.    -   4) In clause C.2.3 (Timing of DU removal and decoding of DU),        the variable prevNonDiscardablePic is specified in a        picture-specific manner, and the value of ph_non_ref_pic_flag is        not taken into account. Consequently, similar problems as above        can occur.    -   5) Similar issues as in problem 4 apply for notDiscardablePic        and prevNonDiscardablePic in clause D.3.2 (Buffering period SEI        message semantics) and prevNonDiscardablePic in clause D.4.2        (Picture timing SEI message semantics).    -   6) The semantics of bp_concatenation_flag and        bp_cpb_removal_delay_delta_minus1 are specified in a        picture-specific manner. However, as HRD operations are        OLS-based, the semantics are technically incorrect and can cause        interoperability problems.    -   7) In the constraint on the value of        bp_alt_cpb_params_present_flag, in the semantics of        bp_alt_cpb_params_present_flag, has two issues: first, it is        specified in a picture-specific manner while it should be        AU-specific, and second, only IRAP pictures are considered,        while GDR pictures also need to be considered.    -   8) The variables BpResetFlag, CpbRemovalDelayMsb[i] and        CpbRemovalDelayVal[i], and the semantics of the syntax elements        pt_dpb_output_delay, pt_dpb_output_du_delay and        pt_display_elemental_periods_minus1, in clause D.4.2 (Picture        timing SEI message semantics), are specified in a        picture-specific manner. However, as HRD operations are        OLS-based, the semantics are technically incorrect and can cause        interoperability problems.    -   9) In clause C.4 (Bitstream conformance), the derivation of        maxPicOrderCnt and minPicOrderCnt does not take into account the        value of ph_non_ref_pic_flag. This would cause problems when        pictures with ph_non_ref_pic_flag equal to 1 are discarded, as        then the derived POC values for pictures and/or the signalled        reference pictures can be wrong, and unexpected incorrect        decoding behaviors, including decoder crash, can occur.    -   10) In the semantics of pt_display_elemental_periods_minus1, the        syntax element fixed_pic_rate_within_cvs_flag[TemporalId] is        used. However, since the semantics should be described in the        context of a target highest TemporalId value, like in other        semantics of BP, PT, and DUI SEI messages,        fixed_pic_rate_within_cvs_flag[Htid] should be used instead.    -   11) The semantics of dui_dpb_output_du_delay are specified in a        picture-specific manner. However, as HRD operations are        OLS-based, the semantics are technically incorrect and can cause        interoperability problems.

5. A LISTING OF TECHNICAL SOLUTIONS AND EMBODIMENTS

To solve the above problems, and others, methods as summarized below aredisclosed. The items should be considered as examples to explain thegeneral concepts and should not be interpreted in a narrow way.Furthermore, these items can be applied individually or combined in anymanner.

-   -   1) To solve problem 1, in clause 3 (Definitions), change the        sentence “RASL pictures are not used as reference pictures for        the decoding process of non-RASL pictures,” in the NOTE as part        of the RASL picture definition to “RASL pictures are not used as        reference pictures for the decoding process of non-RASL        pictures, except that a RADL subpicture, when present, in a RASL        picture may be used for inter prediction of the collocated RADL        subpicture in a RADL picture that is associated with the same        CRA picture as the RASL picture.”    -   2) To solve problem 2, in clause D.4.2 (Picture timing SEI        message semantics), change the description of the constraint on        pt_cpb_alt_timing_info_present_flag being equal to 0 from        picture-specific to AU-specific, and add that the RASL pictures        herein contain only RASL NUTs.        -   a. In one example, the constraint is specified as follows:            When all pictures in the associated AU are RASL pictures            with pps_mixed_nalu_types_in_pic_flag equal to 0, the value            of pt_cpb_alt_timing_info_present_flag shall be equal to 0.        -   b. In another example, the constraint is specified as            follows: When all pictures in the associated AU are RASL            pictures for which pps_mixed_nalu_types_in_pic_flag is equal            to 0, the value of pt_cpb_alt_timing_info_present_flag shall            be equal to 0.        -   c. In yet another example, the constraint is specified as            follows: When all pictures in the associated AU are RASL            pictures containing VCL NAL units all with nal_unit_type            equal to RASL_NUT, the value of            pt_cpb_alt_timing_info_present_flag shall be equal to 0.    -   3) To solve problem 3, add ph_non_ref_pic_flag to the derivation        of prevTid0Pic in the semantics of        delta_poc_msb_cycle_present_flag[i][j] and in the decoding        process for picture order count.    -   4) To solve problem 4, in clause C.2.3 (Timing of DU removal and        decoding of DU), change the specification of        prevNonDiscardablePic from picture-specific to AU-specific,        including renaming it to prevNonDiscardableAu, add        ph_non_ref_pic_flag to the derivation of the same variable.    -   5) To solve problem 5, for notDiscardablePic and        prevNonDiscardablePic in clause D.3.2 (Buffering period SEI        message semantics) and prevNonDiscardablePic in clause D.4.2        (Picture timing SEI message semantics), change the specification        of these variables from picture-specific to AU-specific,        including renaming them to notDiscardableAu and        prevNonDiscardableAu, respectively, and add ph_non_ref_pic_flag        to the derivation of these two variables.    -   6) To solve problem 6, change the description of the semantics        of bp_concatenation_flag and bp_cpb_removal_delay_delta_minus1        from picture-specific to AU-specific.    -   7) To solve problem 7, the constraint on the value of        bp_alt_cpb_params_present_flag is specified in an AU-specific        manner, and it is specified that the value of        bp_alt_cpb_params_present_flag depends additionally on whether        the associated AU is a GDR AU.    -   8) To solve problem 8, change the specification of the variables        BpResetFlag, CpbRemovalDelayMsb[i] and CpbRemovalDelayVal[i] and        the semantics of the syntax elements pt_dpb_output_delay,        pt_dpb_output_du_delay and pt_display_elemental_periods_minus1        in clause D.4.2 (Picture timing SEI message semantics) from        picture-specific to AU-specific.    -   9) To solve problem 9, in clause C.4 (Bitstream conformance),        add ph_non_ref_pic_flag to “The previous picture in decoding        order that has TemporalId equal to 0 and is not a RASL or RADL        picture” in the derivation of maxPicOrderCnt and minPicOrderCnt.    -   10) To solve problem 10, specify the semantics of        pt_display_elemental_periods_minus1 using        fixed_pic_rate_within_cvs_flag[Htid] instead of        fixed_pic_rate_within_cvs_flag[TemporalId].    -   11) To solve problem 11, specify the semantics of        dui_dpb_output_du_delay in an AU-specific manner.

6. EMBODIMENTS

Below are some example embodiments for some of the aspects summarizedabove in Section 5, which can be applied to the VVC specification. Thechanged texts are based on the latest VVC text in JVET-R2001-vA/v10.Most relevant parts that have been added or modified are bolded,underlined and italicized, e.g., “using A

”, and some of the deleted parts are italicized with strikethrough,e.g., “based on

”. There may be some other changes that are editorial in nature and thusnot highlighted.

6.1. First Embodiment

This embodiment is for items 1 to 9.

3 Definitions

. . .

-   -   random access skipped leading (RASL) picture: A coded picture        for which there is at least one VCL NAL unit with nal_unit type        equal to RASL_NUT and other VCL NAL units all have nal_unit type        equal to RASL_NUT or RADL_NUT.        -   NOTE—All RASL pictures are leading pictures of an associated            CRA picture. When the associated CRA picture has            NoOutputBeforeRecoveryFlag equal to 1, the RASL picture is            not output and may not be correctly decodable, as the RASL            picture may contain references to pictures that are not            present in the bitstream. RASL pictures are not used as            reference pictures for the decoding process of non-RASL            pictures,            When sps_field_seq_flag is equal to 0, all RASL pictures,            when present, precede, in decoding order, all non-leading            pictures of the same associated CRA picture.            . . .

7.4.9 Reference Picture Lists Semantics

. . .delta_poc_msb_cycle_present_flag[i][j] equal to 1 specifies thatdelta_poc_msb_cycle_lt[i][j] is present.delta_poc_msb_cycle_present_flag[i][j] equal to 0 specifies thatdelta_poc_msb_cycle_lt[i][j] is not present.Let prevTid0Pic be the previous picture in decoding order that has thesame nuh_layer_id as the current picture, has TemporalId

equal to 0, and is not a RASL or RADL picture. Let setOfPrevPocVals be aset consisting of the following:

-   -   the PicOrderCntVal of prevTid0Pic,    -   the PicOrderCntVal of each picture that is referred to by        entries in RefPicList[0] or RefPicList[1] of prevTid0Pic and has        nuh_layer_id the same as the current picture,    -   the PicOrderCntVal of each picture that follows prevTid0Pic in        decoding order, has nuh_layer_id the same as the current        picture, and precedes the current picture in decoding order.        When there is more than one value in setOfPrevPocVals for which        the value modulo MaxPicOrderCntLsb is equal to PocLsbLt[i][j],        the value of delta_poc_msb_cycle_present_flag[i][j] shall be        equal to 1.        . . .

8.3.1 Decoding Process for Picture Order Count

. . .When ph_poc_msb_cycle_present_flag is equal to 0 and the current pictureis not a CLVSS picture, the variables prevPicOrderCntLsb andprevPicOrderCntMsb are derived as follows:

-   -   Let prevTid0Pic be the previous picture in decoding order that        has the same nuh_layer_id as the current picture, has TemporalId        and        equal to 0, and is not a RASL or RADL picture.    -   The variable prevPicOrderCntLsb is set equal to        ph_pic_order_cnt_lsb of prevTid0Pic.    -   The variable prevPicOrderCntMsb is set equal to PicOrderCntMsb        of prevTid0Pic.        . . .

C.2.3 Timing of DU Removal and Decoding of DU

. . .The nominal removal time of the AU n from the CPB is specified asfollows:

-   -   If AU n is the AU with n equal to 0 (the AU that initializes the        HRD), the nominal removal time of the AU from the CPB is        specified by:

AuNominalRemovalTime[0]=InitCpbRemovalDelay[Htid][ScIdx]+90000  (C.9)

-   -   Otherwise, the following applies:        -   When AU n is the first AU of a BP that does not initialize            the HRD, the following applies:        -   The nominal removal time of the AU n from the CPB is            specified by:

if( !concatenationFlag ) {   baseTime = AuNominalRemovalTime[ first

InPrevBuffPeriod ]   tmpCpbRemovalDelay = AuCpbRemovalDelayVal } else {  baseTime1 = AuNominalRemovalTime prevNonDiscardable

 ]   tmpCpbRemovalDelay1 = (auCpbRemovalDelayDeltaMinus1 + 1 )  baseTime2 = AuNominalRemovalTime[ n − 1 ]   tmpCpbRemovalDelay2=           (C.10)         Ceil( ( InitCpbRemovalDelay[ Htid ] [ ScIdx ]÷ 90000 +   AuFinalArrivalTime[ n − 1 ] − AuNominalRemovalTime[ n − 1 ]) ÷ ClockTick )   if( baseTime1 + ClockTick * tmpCpbRemovalDelay1 <           baseTime2 + ClockTick * tmpCpbRemovalDelay2 ) {      baseTime = baseTime2       tmpCpbRemovalDelay =tmpCpbRemovalDelay2   } else {       baseTime = baseTime1      tmpCpbRemovalDelay = tmpCpbRemovalDelay1   } }

AuNominalRemovalTime[n]=baseTime+(ClockTick*tmpCpbRemovalDelay−CpbDelayOffset)

-   -   -   where AuNominalRemovalTime[first            InPrevBuffPeriod] is the nominal removal time of the first            AU of the previous BP,            AuNominalRemovalTime[prevNonDiscardable            ] is the nominal removal time of            in decoding order with TemporalId equal to 0            that is not a RASL or RADL picture, AuCpbRemovalDelayVal is            the value of CpbRemovalDelayVal[Htid] derived according to            pt_cpb_removal_delay_minus1 [Htid] and            pt_cpb_removal_delay_delta_idx[Htid] in the PT SEI message,            and            bp_cpb_removal_delay_delta_val[pt_cpb_removal_delay_delta_idx[Htid]]            in the BP SEI message, selected as specified in clause C.1,            associated with AU n and concatenationFlag and            CpbRemovalDelayDeltaMinus1 are the values of the syntax            elements bp_concatenation_flag and            bp_cpb_removal_delay_delta_minus1, respectively, in the BP            SEI message, selected as specified in clause C.1, associated            with AU n.        -   After the derivation of the nominal CPB removal time and            before the derivation of the DPB output time of access unit            n, the variables DpbDelayOffset and CpbDelayOffset are            derived as:            -   If one or more of the following conditions are true,                DpbDelayOffset is set equal to the value of the PT SEI                message syntax element dpb_delay_offset[Htid] of AU n+1,                and CpbDelayOffset is set equal to the value of the PT                SEI message syntax element cpb_delay_offset[Htid] of AU                n+1, where the PT SEI message containing the syntax                elements is selected as specified in clause C.1:                -   UseAltCpbParamsFlag for AU n is equal to 1.                -   DefaultInitCpbParamsFlag is equal to 0.            -   Otherwise, DpbDelayOffset and CpbDelayOffset are both                set equal to 0.        -   When AU n is not the first AU of a BP, the nominal removal            time of the AU n from the CPB is specified by:

AuNominalRemovalTime[n]=AuNominalRemovalTime[first

InCurrBuffPeriod]+ClockTick*(AuCpbRemovalDelayVal−CpbDelayOffset)  (C.11)

-   -   -   where AuNominalRemovalTime[first            InCurrBuffPeriod] is the nominal removal time of the first            AU of the current BP and AuCpbRemovalDelayVal is the value            of CpbRemovalDelayVal[OpTid] derived according to            pt_cpb_removal_delay_minus1[OpTid] and            pt_cpb_removal_delay_delta_idx[OpTid] in the PT SEI message,            and            bp_cpb_removal_delay_delta_val[pt_cpb_removal_delay_delta_idx[OpTid]]            in the BP SEI message, selected as specified in clause C.1,            associated with AU n.            . . .

C.4 Bitstream Conformance

Let currPicLayerId be equal to the nuh_layer_id of the current picture.For each current picture, let the variables maxPicOrderCnt andminPicOrderCnt be set equal to the maximum and the minimum,respectively, of the PicOrderCntVal values of the following pictureswith nuh_layer_id equal to currPicLayerId:

-   -   The current picture.    -   The previous picture in decoding order that has TemporalId        equal to 0 and is not a RASL or RADL picture.    -   The STRPs referred to by all entries in RefPicList[0] and all        entries in RefPicList[1] of the current picture.    -   All pictures n that have PictureOutputFlag equal to 1,        AuCpbRemovalTime[n] less than AuCpbRemovalTime[currPic] and        DpbOutputTime[n] greater than or equal to        AuCpbRemovalTime[currPic], where currPic is the current picture.        . . .

D.3.2 Buffering Period SEI Message Semantics

. . .When the BP SEI message is present,

has TemporalId equal to 0 and

that is not a RASL or RADL picture.When

in the bitstream in decoding order, let

in decoding order with TemporalId equal to 0

that is not a RASL or RADL picture.The presence of BP SEI messages is specified as follows:

-   -   If NalHrdBpPresentFlag is equal to 1 or VclHrdBpPresentFlag is        equal to 1, the following applies for each AU in the CVS:        -   If the AU is an IRAP or GDR AU, a BP SEI message applicable            to the operation point shall be associated with the AU.        -   Otherwise, if the AU            a BP SEI message applicable to the operation point may or            may not be associated with the AU.        -   Otherwise, the AU shall not be associated with a BP SEI            message applicable to the operation point.    -   Otherwise (NalHrdBpPresentFlag and VclHrdBpPresentFlag are both        equal to 0), no AU in the CVS shall be associated with a BP SEI        message.    -   NOTE 1—For some applications, frequent presence of BP SEI        messages may be desirable (e.g., for random access at        or for bitstream splicing).        . . .        bp_alt_cpb_params_present_flag equal to 1 specifies the presence        of the syntax element bp_use_alt_cpb_params_flag in the BP SEI        message and the presence of the alternative timing information        in the PT SEI messages in the current BP. When not present, the        value of bp_alt_cpb_params_present_flag is inferred to be equal        to 0. When        the value of bp_alt_cpb_params_present_flag shall be equal to 0.        . . .        bp_concatenation_flag indicates, when        in the bitstream in decoding order, whether the nominal CPB        removal time of        is determined relative to the nominal CPB removal time of the        with a BP SEI message or relative to the nominal CPB removal        time of        .        . . .        bp_cpb_removal_delay_delta_minus1 plus 1, when        in the bitstream in decoding order, specifies a CPB removal        delay increment value relative to the nominal CPB removal time        of        The length of this syntax element is        bp_cpb_removal_delay_length_minus1+1 bits.        When        a BP SEI message and bp_concatenation_flag is equal to 0 and        in the bitstream in decoding order, it is a requirement of        bitstream conformance that the following constraint applies:    -   If        is not associated with a BP SEI message, the        pt_cpb_removal_delay_minus1 of        shall be equal to the pt_cpb_removal_delay_minus1 of        plus bp_cpb_removal_delay_delta_minus1+1.    -   Otherwise, the pt_cpb_removal_delay_minus1        shall be equal to bp_cpb_removal_delay_delta_minus1.    -   NOTE 2—When        a BP SEI message and bp_concatenation_flag is equal to 1, the        pt_cpb_removal_delay_minus1 for        is not used. The above-specified constraint can, under some        circumstances, make it possible to splice bitstreams (that use        suitably-designed referencing structures) by simply changing the        value of bp_concatenation_flag from 0 to 1 in the BP SEI message        for        at the splicing point. When bp_concatenation_flag is equal to 0,        the above-specified constraint enables the decoder to check        whether the constraint is satisfied as a way to detect the loss        of        .        . . .

D.4.2 Picture Timing SEI Message Semantics

. . .pt_cpb_alt_timing_info_present_flag equal to 1 specifies that the syntaxelements pt_nal_cpb_alt_initial_removal_delay_delta[i][j],pt_nal_cpb_alt_initial_removal_offset_delta[i][j],pt_nal_cpb_delay_offset[i], pt_nal_dpb_delay_offset[i],pt_vcl_cpb_alt_initial_removal_delay_delta[i][j],pt_vcl_cpb_alt_initial_removal_offset_delta[i][j],pt_vcl_cpb_delay_offset[i], and pt_vcl_dpb_delay_offset[i] may bepresent in the PT SEI message. pt_cpb_alt_timing_info_present_flag equalto 0 specifies that these syntax elements are not present in the PT SEImessage. When

the value of pt_cpb_alt_timing_info_present_flag shall be equal to 0.

-   -   NOTE 1—The value of pt_cpb_alt_timing_info_present_flag might be        equal to 1 for more than one AU following an        in decoding order. However, the alternative timing is only        applied to the first AU that has        pt_cpb_alt_timing_info_present_flag equal to 1 and follows the        in decoding order.        . . .        pt_vcl_dpb_delay_offset[i] specifies, for the i-th sublayer for        the VCL HRD, an offset to be used in the derivation of the DPB        output times of the IRAP AU associated with the BP SEI message        when the AU associated with the PT SEI message directly follows        in decoding order the IRAP AU associated with the BP SEI        message. The length of pt_vcl_dpb_delay_offset[i] is        bp_dpb_output_delay_length_minus1+1 bits. When not present, the        value of pt_vcl_dpb_delay_offset[i] is inferred to be equal to        0.        The variable BpResetFlag of the        is derived as follows:    -   If        is associated with a BP SEI message, BpResetFlag is set equal to        1.    -   Otherwise, BpResetFlag is set equal to 0.        . . .        pt_cpb_removal_delay_delta_idx[i] specifies the index of the CPB        removal delta that applies to Htid equal to i in the list of        bp_cpb_removal_delay_delta_val[j] for j ranging from 0 to        bp_num_cpb_removal_delay_deltas_minus1, inclusive. The length of        pt_cpb_removal_delay_delta_idx[i] is        Ceil(Log2(bp_num_cpb_removal_delay_deltas_minus1+1)) bits. When        pt_cpb_removal_delay_delta_idx[i] is not present and        pt_cpb_removal_delay_delta_enabled_flag[i] is equal to 1, the        value of pt_cpb_removal_delay_delta_idx[i] is inferred to be        equal to 0.        The variables CpbRemovalDelayMsb[i] and CpbRemovalDelayVal[i] of        are derived as follows:    -   If the current AU is the AU that initializes the HRD,        CpbRemovalDelayMsb[i] and CpbRemovalDelayVal[i] are both set        equal to 0, and the value of cpbRemovalDelayValTmp[i] is set        equal to pt_cpb_removal_delay_minus1[i]+1.    -   Otherwise, let        in decoding order        TemporalId equal to 0        that is not a RASL or RADL picture, let        prevCpbRemovalDelayMinus1[i], prevCpbRemovalDelayMsb[i], and        prevBpResetFlag be set equal to the values of        cpbRemovalDelayValTmp[i]−1, CpbRemovalDelayMsb[i], and        BpResetFlag, respectively, for        , and the following applies:        . . .        pt_dpb_output_delay is used to compute the DPB output time of        . It specifies how many clock ticks to wait after removal of an        AU from the CPB before        output from the DPB.    -   NOTE 2—A decoded picture is not removed from the DPB at its        output time when it is still marked as “used for short-term        reference” or “used for long-term reference”.        The length of pt_dpb_output_delay is        bp_dpb_output_delay_length_minus1+1 bits. When        max_dec_pic_buffering_minus1[Htid] is equal to 0, the value of        pt_dpb_output_delay shall be equal to 0. The output time derived        from the pt_dpb_output_delay of any picture that is output from        an output timing conforming decoder shall precede the output        time derived from the pt_dpb_output_delay of all pictures in any        subsequent CVS in decoding order.        The picture output order established by the values of this        syntax element shall be the same order as established by the        values of PicOrderCntVal.        For pictures that are not output by the “bumping” process        because they precede, in decoding order,        that has ph_no_output_of_prior_pics_flag equal to 1 or inferred        to be equal to 1, the output times derived from        pt_dpb_output_delay shall be increasing with increasing value of        PicOrderCntVal relative to all pictures within the same CVS.        pt_dpb_output_du_delay is used to compute the DPB output time of        when DecodingUnitHrdFlag is equal to 1. It specifies how many        sub clock ticks to wait after removal of the last DU in an AU        from the CPB before        output from the DPB.        . . .        pt_display_elemental_periods_minus1 plus 1, when        sps_field_seq_flag is equal to 0 and        fixed_pic_rate_within_cvs_flag[        ] is equal to 1, indicates the number of elemental picture        period intervals that        occupies for the display model.        When fixed_pic_rate_within_cvs_flag[        ] is equal to 0 or sps_field_seq_flag is equal to 1, the value        of pt_display_elemental_periods_minus1 shall be equal to 0.        When sps_field_seq_flag is equal to 0 and        fixed_pic_rate_within_cvs_flag[        ] is equal to 1, a value of pt_display_elemental_periods_minus1        greater than 0 may be used to indicate a frame repetition period        for displays that use a fixed frame refresh interval equal to        DpbOutputElementalInterval[n] as given by Equation 112.        . . .

D.5.2 DU Information SEI Message Semantics

. . .dui_dpb_output_du_delay is used to compute the DPB output time of

when DecodingUnitHrdFlag is equal to 1 andbp_du_dpb_params_in_pic_timing_sei_flag is equal to 0. It specifies howmany sub clock ticks to wait after removal of the last DU in an AU fromthe CPB before

output from the DPB. When not present, the value ofdui_dpb_output_du_delay is inferred to be equal topt_dpb_output_du_delay. The length of the syntax elementdui_dpb_output_du_delay is given in bits bybp_dpb_output_delay_du_length_minus1+1.It is a requirement of bitstream conformance that all DU information SEImessages that are associated with the same AU, apply to the sameoperation point, and have bp_du_dpb_params_in_pic_timing_sei_flag equalto 0 shall have the same value of dui_dpb_output_du_delay.The output time derived from the dui_dpb_output_du_delay of any picturethat is output from an output timing conforming decoder shall precedethe output time derived from the dui_dpb_output_du_delay of all picturesin any subsequent CVS in decoding order.The picture output order established by the values of this syntaxelement shall be the same order as established by the values ofPicOrderCntVal.For pictures that are not output by the “bumping” process because theyprecede, in decoding order, a

CVSS

that has ph_no_output_of_prior_pics_flag equal to 1 or inferred to beequal to 1, the output times derived from dui_dpb_output_du_delay shallbe increasing with increasing value of PicOrderCntVal relative to allpictures within the same CVS.For any two pictures in the CVS, the difference between the output timesof the two pictures when DecodingUnitHrdFlag is equal to 1 shall beidentical to the same difference when DecodingUnitHrdFlag is equal to 0.. . .

FIG. 1 is a block diagram showing an example video processing system1000 in which various embodiments disclosed herein may be implemented.Various implementations may include some or all of the components of thesystem 1000. The system 1000 may include input 1002 for receiving videocontent. The video content may be received in a raw or uncompressedformat, e.g., 8- or 10-bit multi-component pixel values, or may be in acompressed or encoded format. The input 1002 may represent a networkinterface, a peripheral bus interface, or a storage interface. Examplesof network interface include wired interfaces such as Ethernet, passiveoptical network (PON), etc. and wireless interfaces such as Wi-Fi orcellular interfaces.

The system 1000 may include a coding component 1004 that may implementthe various coding or encoding methods described in the presentdisclosure. The coding component 1004 may reduce the average bitrate ofvideo from the input 1002 to the output of the coding component 1004 toproduce a coded representation of the video. The coding techniques aretherefore sometimes called video compression or video transcodingtechniques. The output of the coding component 1004 may be eitherstored, or transmitted via a communication connected, as represented bythe component 1006. The stored or communicated bitstream (or coded)representation of the video received at the input 1002 may be used bythe component 1008 for generating pixel values or displayable video thatis sent to a display interface 1010. The process of generatinguser-viewable video from the bitstream representation is sometimescalled video decompression. Furthermore, while certain video processingoperations are referred to as “coding” operations or tools, it will beappreciated that the coding tools or operations are used at an encoderand corresponding decoding tools or operations that reverse the resultsof the coding will be performed by a decoder.

Examples of a peripheral bus interface or a display interface mayinclude universal serial bus (USB) or high definition multimediainterface (HDMI) or DisplayPort, and so on. Examples of storageinterfaces include serial advanced technology attachment (SATA),peripheral component interface (PCI), integrated drive electronics (IDE)interface, and the like. The embodiments described in the presentdisclosure may be embodied in various electronic devices such as mobilephones, laptops, smartphones or other devices that are capable ofperforming digital data processing and/or video display.

FIG. 2 is a block diagram of a video processing apparatus 2000. Theapparatus 2000 may be used to implement one or more of the methodsdescribed herein. The apparatus 2000 may be embodied in a smartphone,tablet, computer, Internet of Things (IoT) receiver, and so on. Theapparatus 2000 may include one or more processors 2002, one or morememories 2004 and video processing hardware 2006. The processor(s) 2002may be configured to implement one or more methods described in thepresent disclosure (e.g., in FIGS. 6-9 ). The memory (memories) 2004 maybe used for storing data and code used for implementing the methods andembodiments described herein. The video processing hardware 2006 may beused to implement, in hardware circuitry, some embodiments described inthe present disclosure. In some embodiments, the hardware 2006 may bepartly or entirely in the one or more processors 2002, e.g., a graphicsprocessor.

FIG. 3 is a block diagram that illustrates an example video codingsystem 100 that may utilize the embodiments of this disclosure. As shownin FIG. 3 , video coding system 100 may include a source device 110 anda destination device 120. Source device 110 generates encoded video datawhich may be referred to as a video encoding device. Destination device120 may decode the encoded video data generated by source device 110which may be referred to as a video decoding device. Source device 110may include a video source 112, a video encoder 114, and an input/output(I/O) interface 116.

Video source 112 may include a source such as a video capture device, aninterface to receive video data from a video content provider, and/or acomputer graphics system for generating video data, or a combination ofsuch sources. The video data may comprise one or more pictures. Videoencoder 114 encodes the video data from video source 112 to generate abitstream. The bitstream may include a sequence of bits that form acoded representation of the video data. The bitstream may include codedpictures and associated data. The coded picture is a codedrepresentation of a picture. The associated data may include sequenceparameter sets, picture parameter sets, and other syntax structures. I/Ointerface 116 may include a modulator/demodulator (modem) and/or atransmitter. The encoded video data may be transmitted directly todestination device 120 via I/O interface 116 through network 130 a. Theencoded video data may also be stored onto a storage medium/server 130 bfor access by destination device 120.

Destination device 120 may include an I/O interface 126, a video decoder124, and a display device 122.

I/O interface 126 may include a receiver and/or a modem. I/O interface126 may acquire encoded video data from the source device 110 or thestorage medium/server 130 b. Video decoder 124 may decode the encodedvideo data. Display device 122 may display the decoded video data to auser. Display device 122 may be integrated with the destination device120, or may be external to destination device 120 which be configured tointerface with an external display device.

Video encoder 114 and video decoder 124 may operate according to a videocompression standard, such as the High Efficiency Video Coding (HEVC)standard, Versatile Video Coding (VVM) standard and other current and/orfurther standards.

FIG. 4 is a block diagram illustrating an example of video encoder 200,which may be video encoder 114 in the system 100 illustrated in FIG. 3 .

Video encoder 200 may be configured to perform any or all of theembodiments of this disclosure. In the example of FIG. 4 , video encoder200 includes a plurality of functional components. The embodimentsdescribed in this disclosure may be shared among the various componentsof video encoder 200. In some examples, a processor may be configured toperform any or all of the embodiments described in this disclosure.

The functional components of video encoder 200 may include a partitionunit 201, a prediction unit 202 which may include a mode select unit203, a motion estimation unit 204, a motion compensation unit 205, andan intra prediction unit 206; a residual generation unit 207; atransform unit 208; a quantization unit 209; an inverse quantizationunit 210; an inverse transform unit 211; a reconstruction unit 212; abuffer 213; and an entropy encoding unit 214.

In other examples, video encoder 200 may include more, fewer, ordifferent functional components. In an example, prediction unit 202 mayinclude an intra block copy (IBC) unit. The IBC unit may performprediction in an IBC mode in which at least one reference picture is apicture where the current video block is located.

Furthermore, some components, such as motion estimation unit 204 andmotion compensation unit 205 may be highly integrated, but arerepresented in the example of FIG. 4 separately for purposes ofexplanation.

Partition unit 201 may partition a picture into one or more videoblocks. Video encoder 200 and video decoder 300 may support variousvideo block sizes.

Mode select unit 203 may select one of the coding modes, intra or inter,e.g., based on error results, and provide the resulting intra- orinter-coded block to a residual generation unit 207 to generate residualblock data and to a reconstruction unit 212 to reconstruct the encodedblock for use as a reference picture. In some example, mode select unit203 may select a combination of intra and inter prediction (CIIP) modein which the prediction is based on an inter prediction signal and anintra prediction signal. Mode select unit 203 may also select aresolution for a motion vector (e.g., a sub-pixel or integer pixelprecision) for the block in the case of inter prediction.

To perform inter prediction on a current video block, motion estimationunit 204 may generate motion information for the current video block bycomparing one or more reference frames from buffer 213 to the currentvideo block. Motion compensation unit 205 may determine a predictedvideo block for the current video block based on the motion informationand decoded samples of pictures from buffer 213 other than the pictureassociated with the current video block.

Motion estimation unit 204 and motion compensation unit 205 may performdifferent operations for a current video block, for example, dependingon whether the current video block is in an I slice, a P slice, or a Bslice.

In some examples, motion estimation unit 204 may perform uni-directionalprediction for the current video block, and motion estimation unit 204may search reference pictures of list 0 or list 1 for a reference videoblock for the current video block. Motion estimation unit 204 may thengenerate a reference index that indicates the reference picture in list0 or list 1 that contains the reference video block and a motion vectorthat indicates a spatial displacement between the current video blockand the reference video block. Motion estimation unit 204 may output thereference index, a prediction direction indicator, and the motion vectoras the motion information of the current video block. Motioncompensation unit 205 may generate the predicted video block of thecurrent block based on the reference video block indicated by the motioninformation of the current video block.

In other examples, motion estimation unit 204 may perform bi-directionalprediction for the current video block, motion estimation unit 204 maysearch the reference pictures in list 0 for a reference video block forthe current video block and may also search the reference pictures inlist 1 for another reference video block for the current video block.Motion estimation unit 204 may then generate reference indexes thatindicate the reference pictures in list 0 and list 1 containing thereference video blocks and motion vectors that indicate spatialdisplacements between the reference video blocks and the current videoblock. Motion estimation unit 204 may output the reference indexes andthe motion vectors of the current video block as the motion informationof the current video block. Motion compensation unit 205 may generatethe predicted video block of the current video block based on thereference video blocks indicated by the motion information of thecurrent video block.

In some examples, motion estimation unit 204 may output a full set ofmotion information for decoding processing of a decoder.

In some examples, motion estimation unit 204 may not output a full setof motion information for the current video. Rather, motion estimationunit 204 may signal the motion information of the current video blockwith reference to the motion information of another video block. Forexample, motion estimation unit 204 may determine that the motioninformation of the current video block is sufficiently similar to themotion information of a neighboring video block.

In one example, motion estimation unit 204 may indicate, in a syntaxstructure associated with the current video block, a value thatindicates to the video decoder 300 that the current video block has thesame motion information as the another video block.

In another example, motion estimation unit 204 may identify, in a syntaxstructure associated with the current video block, another video blockand a motion vector difference (MVD). The motion vector differenceindicates a difference between the motion vector of the current videoblock and the motion vector of the indicated video block. The videodecoder 300 may use the motion vector of the indicated video block andthe motion vector difference to determine the motion vector of thecurrent video block.

As discussed above, video encoder 200 may predictively signal the motionvector. Two examples of predictive signalling techniques that may beimplemented by video encoder 200 include advanced motion vectorprediction (AMVP) and merge mode signalling.

Intra prediction unit 206 may perform intra prediction on the currentvideo block. When intra prediction unit 206 performs intra prediction onthe current video block, intra prediction unit 206 may generateprediction data for the current video block based on decoded samples ofother video blocks in the same picture. The prediction data for thecurrent video block may include a predicted video block and varioussyntax elements.

Residual generation unit 207 may generate residual data for the currentvideo block by subtracting (e.g., indicated by the minus sign) thepredicted video block(s) of the current video block from the currentvideo block. The residual data of the current video block may includeresidual video blocks that correspond to different sample components ofthe samples in the current video block.

In other examples, there may be no residual data for the current videoblock for the current video block, for example in a skip mode, andresidual generation unit 207 may not perform the subtracting operation.

Transform processing unit 208 may generate one or more transformcoefficient video blocks for the current video block by applying one ormore transforms to a residual video block associated with the currentvideo block.

After transform processing unit 208 generates a transform coefficientvideo block associated with the current video block, quantization unit209 may quantize the transform coefficient video block associated withthe current video block based on one or more quantization parameter (QP)values associated with the current video block.

Inverse quantization unit 210 and inverse transform unit 211 may applyinverse quantization and inverse transforms to the transform coefficientvideo block, respectively, to reconstruct a residual video block fromthe transform coefficient video block. Reconstruction unit 212 may addthe reconstructed residual video block to corresponding samples from oneor more predicted video blocks generated by the prediction unit 202 toproduce a reconstructed video block associated with the current blockfor storage in the buffer 213.

After reconstruction unit 212 reconstructs the video block, loopfiltering operation may be performed reduce video blocking artifacts inthe video block.

Entropy encoding unit 214 may receive data from other functionalcomponents of the video encoder 200. When entropy encoding unit 214receives the data, entropy encoding unit 214 may perform one or moreentropy encoding operations to generate entropy encoded data and outputa bitstream that includes the entropy encoded data.

FIG. 5 is a block diagram illustrating an example of video decoder 300which may be video decoder 124 in the system 100 illustrated in FIG. 3 .

The video decoder 300 may be configured to perform any or all of theembodiments of this disclosure. In the example of FIG. 5 , the videodecoder 300 includes a plurality of functional components. Theembodiments described in this disclosure may be shared among the variouscomponents of the video decoder 300. In some examples, a processor maybe configured to perform any or all of the embodiments described in thisdisclosure.

In the example of FIG. 5 , video decoder 300 includes an entropydecoding unit 301, a motion compensation unit 302, an intra predictionunit 303, an inverse quantization unit 304, an inverse transformationunit 305, a reconstruction unit 306, and a buffer 307. Video decoder 300may, in some examples, perform a decoding pass generally reciprocal tothe encoding pass described with respect to video encoder 200 (FIG. 4 ).

Entropy decoding unit 301 may retrieve an encoded bitstream. The encodedbitstream may include entropy coded video data (e.g., encoded blocks ofvideo data). Entropy decoding unit 301 may decode the entropy codedvideo data, and from the entropy decoded video data, motion compensationunit 302 may determine motion information including motion vectors,motion vector precision, reference picture list indexes, and othermotion information. Motion compensation unit 302 may, for example,determine such information by performing the AMVP and merge mode.

Motion compensation unit 302 may produce motion compensated blocks,possibly performing interpolation based on interpolation filters.Identifiers for interpolation filters to be used with sub-pixelprecision may be included in the syntax elements.

Motion compensation unit 302 may use interpolation filters as used byvideo encoder 200 during encoding of the video block to calculateinterpolated values for sub-integer pixels of a reference block. Motioncompensation unit 302 may determine the interpolation filters used byvideo encoder 200 according to received syntax information and use theinterpolation filters to produce predictive blocks.

Motion compensation unit 302 may use some of the syntax information todetermine sizes of blocks used to encode frame(s) and/or slice(s) of theencoded video sequence, partition information that describes how eachmacroblock of a picture of the encoded video sequence is partitioned,modes indicating how each partition is encoded, one or more referenceframes (and reference frame lists) for each inter-encoded block, andother information to decode the encoded video sequence.

Intra prediction unit 303 may use intra prediction modes for examplereceived in the bitstream to form a prediction block from spatiallyadjacent blocks. Inverse quantization unit 304 inverse quantizes, i.e.,de-quantizes, the quantized video block coefficients provided in thebitstream and decoded by entropy decoding unit 301. Inversetransformation unit 305 applies an inverse transform.

Reconstruction unit 306 may sum the residual blocks with thecorresponding prediction blocks generated by motion compensation unit302 or intra prediction unit 303 to form decoded blocks. If desired, adeblocking filter may also be applied to filter the decoded blocks inorder to remove blockiness artifacts. The decoded video blocks are thenstored in buffer 307, which provides reference blocks for subsequentmotion compensation/intra prediction and also produces decoded video forpresentation on a display device.

FIGS. 6-16 show example methods that can implement the embodimentsdescribed above in, for example, the embodiments shown in FIGS. 1-5 .

FIG. 6 shows a flowchart for an example method 600 of video processing.The method 600 includes, at operation 610, performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, the bitstream conforming to a format rule that specifies that apicture timing (PT) supplemental enhancement information (SEI) message,when included in the bitstream, is access unit (AU) specific, and eachpicture of the one or more pictures that is a random access skippedleading (RASL) picture including only a RASL network abstraction layerunit type (NUT).

FIG. 7 shows a flowchart for an example method 700 of video processing.The method 700 includes, at operation 710, performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, the bitstream conforming to a format rule that permits a use of arandom access decodable leading (RADL) subpicture in a random accessskipped leading (RASL) picture as a reference subpicture for predictinga collocated RADL picture in a RADL picture associated with a same cleanrandom access (CRA) picture as the RASL picture.

FIG. 8 shows a flowchart for an example method 800 of video processing.The method 800 includes, at operation 810, performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, the bitstream conforming to a format rule that specifies that aderivation of a picture associated with a first flag and in a decodingprocess for a picture order count is based on a second flag, the pictureassociated with the first flag being a previous picture in a decodingorder that has (i) a first identifier same as a slice or picture headerreferring to a reference picture list syntax structure, (ii) a secondidentifier and the second flag being equal to zero, and (iii) a picturetype different from a random access skipped leading (RASL) picture and arandom access decodable leading (RADL) picture, the first flagindicating whether a third flag is present in the bitstream, the secondflag indicating whether a current picture is used as a referencepicture, and the third flag being used to determine a value of one ormore most significant bits of a picture order count value of a long-termreference picture.

FIG. 9 shows a flowchart for an example method 900 of video processing.The method 900 includes, at operation 910, performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, the bitstream conforming to a format rule that specifies that avariable, which is used to determine a timing of a removal of a decodingunit (DU) or decoding the DU, being access unit (AU) specific andderived based on a flag that indicates whether a current picture isallowed to be used as a reference picture.

FIG. 10 shows a flowchart for an example method 1000 of videoprocessing. The method 1000 includes, at operation 1010, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a buffering period supplemental enhancement information(SEI) message and a picture timing SEI message, when included in thebitstream, are access unit (AU) specific, a first variable associatedwith the buffering period SEI message and a second variable associatedwith the buffering period SEI message and the picture timing SEI messagebeing derived based on a flag that indicates whether a current pictureis allowed to be used as a reference picture, the first variable beingindicative of an access unit comprising (i) an identifier that is equalto zero and (ii) a picture that is not a random access skipped leading(RASL) picture or a random access decodable leading (RADL) picture andfor which the flag is equal to zero, and the second variable beingindicative of a current AU not being a first AU in a decoding order anda previous AU in the decoding order comprising (i) the identifier thatis equal to zero and (ii) a picture that is not a random access skippedleading (RASL) picture or a random access decodable leading (RADL)picture and for which the flag is equal to zero.

FIG. 11 shows a flowchart for an example method 1100 of videoprocessing. The method 1100 includes, at operation 1110, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a derivation of a first variable and a second variableassociated with a first picture and a second picture is based on a flag,the first picture being a current picture and the second picture being aprevious picture in a decoding order that (i) comprises a firstidentifier that is equal to zero, (ii) comprises a flag that is equal tozero, and (iii) is not a random access skipped leading (RASL) picture ora random access decodable leading (RADL) picture, and the first variableand the second variable being a maximum value and a minimum value,respectively, of a picture order count of each of the following pictureswith a second identifier that is equal to that of the first picture (i)the first picture, (ii) the second picture, (iii) one or more short-termreference pictures referred to by all entries in reference picture listsof the first picture, and (iv) each picture that has been output with acoded picture buffer (CPB) removal time less than the CPB removal timeof the first picture and a decoded picture buffer (DPB) output timegreater than or equal to the CPB removal time of the first picture.

FIG. 12 shows a flowchart for an example method 1200 of videoprocessing. The method 1200 includes, at operation 1210, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a flag and a syntax element, when included in thebitstream, are access unit (AU) specific, the flag indicating, inresponse to a current AU not being a first AU in the bitstream in adecoding order, whether a nominal coded picture buffer (CPB) removaltime of the current AU is determined relative to (a) a nominal CPBremoval time of a previous AU associated with a buffering periodsupplemental enhancement information (SEI) message or (b) a nominal CPBremoval time of the current AU, and the syntax element specifying, inresponse to a current AU not being a first AU in the bitstream in adecoding order, a CPB removal delay increment value relative to thenominal CPB removal time of the current AU.

FIG. 13 shows a flowchart for an example method 1300 of videoprocessing. The method 1300 includes, at operation 1310, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a plurality of variables and a picture timingsupplemental enhancement information (SEI) message, when included in thebitstream, are access unit (AU) specific, the picture timing SEI messagecomprising a plurality of syntax elements, a first variable of theplurality of variables indicating whether a current AU is associatedwith a buffering period SEI message, a second variable and a thirdvariable of the plurality of variables being associated with anindication of whether the current AU is an AU that initializes ahypothetical reference decoder (HRD), a first syntax element of theplurality of syntax elements specifying a number of clock ticks to waitafter a removal of an AU from a coded picture buffer (CPB) before one ormore decoded pictures of the AU are output from the decoded picturebuffer (DPB), a second syntax element of the plurality of syntaxelements specifying a number of sub clock ticks to wait after a removalof a last decoding unit (DU) in an AU from the CPB before the one ormore decoded pictures of the AU are output from the DPB, and a thirdsyntax element of the plurality of syntax elements specifying a numberof elemental picture period intervals that one or more decoded picturesof the current AU occupy for a display model.

FIG. 14 shows a flowchart for an example method 1400 of videoprocessing. The method 1400 includes, at operation 1410, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a syntax element associated with a decoding picturebuffer (DPB), when included in the bitstream, is access unit (AU)specific, and the syntax element specifying a number of sub clock ticksto wait after removal of a last decoding unit (DU) in an AU from thecoded picture buffer (CPB) before one or more decoded pictures of the AUare output from the DPB.

FIG. 15 shows a flowchart for an example method 1500 of videoprocessing. The method 1500 includes, at operation 1510, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a flag, when included in the bitstream, is access unit(AU) specific, the value of the flag being based on whether anassociated AU is an intra random access points (IRAP) AU or a gradualdecoding refresh (GDR) AU, and the value of the flag specifying (i)whether a syntax element is present in a buffering period supplementalenhancement information (SEI) message and (ii) whether alternativetiming information is present in a picture timing SEI message in acurrent buffering period.

FIG. 16 shows a flowchart for an example method 1600 of videoprocessing. The method 1600 includes, at operation 1610, performing aconversion between a video comprising one or more pictures and abitstream of the video, the bitstream conforming to a format rule thatspecifies that a value of a first syntax element is based on a flag thatindicates whether a temporal distance between output times ofconsecutive pictures of a hypothetical reference decoder (HRD) isconstrained and a variable that identifies a highest temporal sublayerto be decoded, and the first syntax element specifying a number ofelemental picture period intervals that one or more decoded pictures ofthe current AU occupy for a display model.

A listing of solutions preferred by some embodiments is provided next.

A1. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a picture timing (PT) supplementalenhancement information (SEI) message, when included in the bitstream,is access unit (AU) specific, and wherein each picture of the one ormore pictures that is a random access skipped leading (RASL) pictureincludes only a RASL network abstraction layer unit type (NUT).

A2. The method of solution A1, wherein, in response to each picture inan associated AU being a RASL picture with a first flag equaling zero, asecond flag is equal to zero, wherein the first flag indicates whethereach picture referring to a picture parameter set (PPS) has more thanone video coding layer (VCL) network abstraction layer (NAL) unit and atleast two of the more than one VCL NAL units are of different types, andwherein the second flag indicates whether one or more syntax elementsrelated to timing information are allowed to be present in the picturetiming SEI message.

A3. The method of solution A2, wherein the first flag ispps_mixed_nalu_types_in_pic_flag and the second flag ispt_cpb_alt_timing_info_present_flag.

A4. The method of solution A2, wherein the one or more syntax elementscomprises at least one of a first syntax element indicating analternative initial coded picture buffer (CPB) removal delay delta foran i-th sublayer for a j-th CPB for a NAL hypothetical reference decoder(HRD) in units of a 90 kHz clock, a second syntax element indicating analternative initial CPB removal offset delta for the i-th sublayer forthe j-th CPB for the NAL HRD in units of a 90 kHz clock, a third syntaxelement indicating, for the i-th sublayer for the NAL HRD, an offset tobe used in a derivation of a nominal CPB removal time of an AUassociated with the PT SEI message and of one or more subsequent AUs ina decoding order, when the AU associated with the PT SEI messagedirectly follows, in the decoding order, an AU associated with abuffering period (BP) SEI message, a fourth syntax element indicating,for the i-th sublayer for the NAL HRD, an offset to be used in aderivation of a decoded picture buffer (DPB) output time of an intrarandom access point (IRAP) AU associated with the BP SEI message whenthe AU associated with the PT SEI message directly follows, in thedecoding order, the IRAP AU associated with the BP SEI message, a fifthsyntax element indicating an alternative initial CPB removal delay deltafor an i-th sublayer for a j-th CPB for a VCL HRD in units of a 90 kHzclock, a sixth syntax element indicating alternative initial CPB removaloffset delta for the i-th sublayer for the j-th CPB for the VCL HRD inunits of a 90 kHz clock, a seventh syntax element indicating, for thei-th sublayer for the VCL HRD, an offset to be used in the derivation ofthe nominal CPB removal time of the AU associated with the PT SEImessage and of the one or more subsequent AUs in a decoding order, whenthe AU associated with the PT SEI message directly follows, in thedecoding order, the AU associated with the BP SEI message, an eighthsyntax element indicating for the i-th sublayer for the VCL HRD, anoffset to be used in the derivation of the DPB output time of the IRAPAU associated with the BP SEI message when the AU associated with the PTSEI message directly follows, in the decoding order, the IRAP AUassociated with the BP SEI message.

A5. The method of solution A1, wherein, in response to each picture inan associated AU being a RASL picture including video coding layer (VCL)network abstraction layer (NAL) units with each VCL NAL unit being aRASL NUT, a flag is equal to zero, wherein the flag indicates whetherone or more syntax elements related to timing information are present inthe picture timing SEI message.

A6. The method of solution A5, wherein the flag ispt_cpb_alt_timing_info_present_flag.

A7. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule permits a use of a random access decodable leading (RADL)subpicture in a random access skipped leading (RASL) picture as areference subpicture for predicting a collocated RADL picture in a RADLpicture associated with a same clean random access (CRA) picture as theRASL picture.

Another listing of solutions preferred by some embodiments is providednext.

B1. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a derivation of a picture associated with afirst flag and in a decoding process for a picture order count is basedon a second flag, wherein the picture associated with the first flag isa previous picture in a decoding order that has (i) a first identifiersame as a slice or picture header referring to a reference picture listsyntax structure, (ii) a second identifier and the second flag beingequal to zero, and (iii) a picture type different from a random accessskipped leading (RASL) picture and a random access decodable leading(RADL) picture, wherein the first flag indicates whether a third flag ispresent in the bitstream, wherein the second flag indicates whether acurrent picture is used as a reference picture, and wherein the thirdflag is used to determine a value of one or more most significant bitsof a picture order count value of a long-term reference picture.

B2. The method of solution B1, wherein the first identifier is anidentifier of a layer and the second identifier is a temporalidentifier.

B3. The method of solution B1, wherein the first identifier is a syntaxelement and the second identifier is a variable.

B4. The method of any of solutions B1 to B3, wherein the first flag isdelta_poc_msb_cycle_present_flag, the second flag isph_non_ref_pic_flag, and the third flag isdelta_poc_msb_cycle_present_flag, and wherein the first identifier isnuh_layer_id and the second identifier is TemporalId.

B5. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a variable, which is used to determine atiming of a removal of a decoding unit (DU) or decoding the DU, isaccess unit (AU) specific and derived based on a flag that indicateswhether a current picture is allowed to be used as a reference picture.

B6. The method of solution B5, wherein the variable isprevNonDiscardableAu and the flag is ph_non_ref_pic_flag.

B7. The method of solution B6, wherein ph_non_ref_pic_flag equaling onespecifies that the current picture is never used as the referencepicture.

B8. The method of solution B6, wherein ph_non_ref_pic_flag equaling zerospecifies that the current picture might or might not be used as thereference picture.

B9. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a buffering period supplemental enhancementinformation (SEI) message and a picture timing SEI message, whenincluded in the bitstream, are access unit (AU) specific, wherein afirst variable associated with the buffering period SEI message and asecond variable associated with the buffering period SEI message and thepicture timing SEI message are derived based on a flag that indicateswhether a current picture is allowed to be used as a reference picture,wherein the first variable is indicative of an access unit comprising(i) an identifier that is equal to zero and (ii) a picture that is not arandom access skipped leading (RASL) picture or a random accessdecodable leading (RADL) picture and for which the flag is equal tozero, and wherein the second variable is indicative of a current AU notbeing a first AU in a decoding order and a previous AU in the decodingorder comprising (i) the identifier that is equal to zero and (ii) apicture that is not a random access skipped leading (RASL) picture or arandom access decodable leading (RADL) picture and for which the flag isequal to zero.

B10. The method of solution B9, wherein the identifier is a temporalidentifier.

B11. The method of solution B9, wherein the first variable isnotDiscardableAu, the second variable is prevNonDiscardableAu, the flagis ph_non_ref_pic_flag, and the identifier is TemporalId.

B12. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a derivation of a first variable and a secondvariable associated with a first picture and a second picture is basedon a flag, wherein the first picture is a current picture and the secondpicture is a previous picture in a decoding order that (i) comprises afirst identifier that is equal to zero, (ii) comprises a flag that isequal to zero, and (iii) is not a random access skipped leading (RASL)picture or a random access decodable leading (RADL) picture, and whereinthe first variable and the second variable are a maximum value and aminimum value, respectively, of a picture order count of each of thefollowing pictures with a second identifier that is equal to that of thefirst picture (i) the first picture, (ii) the second picture, (iii) oneor more short-term reference pictures referred to by all entries inreference picture lists of the first picture, and (iv) each picture thathas been output with a coded picture buffer (CPB) removal time less thanthe CPB removal time of the first picture and a decoded picture buffer(DPB) output time greater than or equal to the CPB removal time of thefirst picture.

B13. The method of solution B12, wherein the first variable indicatesthe maximum value of the picture order count and the second variableindicates the minimum value of the picture order count.

B14. The method of solution B12, wherein the flag indicates whether thecurrent picture is allowed to be used as a reference picture.

B15. The method of solution B12, wherein the first identifier is atemporal identifier and the second identifier is an identifier of alayer.

B16. The method of any of solutions B12 to B15, wherein the firstvariable is maxPicOrderCnt, the second variable is minPicOrderCnt, thefirst identifier is TemporalId, the second identifier is nuh_layer_id,and the flag is ph_non_ref_pic_flag.

Yet another listing of solutions preferred by some embodiments isprovided next.

C1. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a flag and a syntax element, when included inthe bitstream, are access unit (AU) specific, wherein the flagindicates, in response to a current AU not being a first AU in thebitstream in a decoding order, whether a nominal coded picture buffer(CPB) removal time of the current AU is determined relative to (a) anominal CPB removal time of a previous AU associated with a bufferingperiod supplemental enhancement information (SEI) message or (b) anominal CPB removal time of the current AU, and wherein the syntaxelement specifies, in response to a current AU not being a first AU inthe bitstream in a decoding order, a CPB removal delay increment valuerelative to the nominal CPB removal time of the current AU.

C2. The method of solution C1, wherein a length of the syntax element isindicated in a syntax structure of the buffering period SEI message.

C3. The method of solution C1, wherein a length of the syntax element is(bp_cpb_removal_delay_length_minus1+1) bits.

C4. The method of any of solutions C1 to C3, wherein the flag isbp_concatenation_flag and the syntax element isbp_cpb_removal_delay_delta_minus1.

C5. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a plurality of variables and a picture timingsupplemental enhancement information (SEI) message, when included in thebitstream, are access unit (AU) specific, wherein the picture timing SEImessage comprises a plurality of syntax elements, wherein a firstvariable of the plurality of variables indicates whether a current AU isassociated with a buffering period SEI message, wherein a secondvariable and a third variable of the plurality of variables areassociated with an indication of whether the current AU is an AU thatinitializes a hypothetical reference decoder (HRD), wherein a firstsyntax element of the plurality of syntax elements specifies a number ofclock ticks to wait after a removal of an AU from a coded picture buffer(CPB) before one or more decoded pictures of the AU are output from thedecoded picture buffer (DPB), wherein a second syntax element of theplurality of syntax elements specifies a number of sub clock ticks towait after a removal of a last decoding unit (DU) in an AU from the CPBbefore the one or more decoded pictures of the AU are output from theDPB, and wherein a third syntax element of the plurality of syntaxelements specifies a number of elemental picture period intervals thatone or more decoded pictures of the current AU occupy for a displaymodel.

C6. The method of solution C5, wherein the first variable isBpResetFlag, the second variable is CpbRemovalDelayMsb, and the thirdvariable is CpbRemovalDelayVal.

C7. The method of solution C5, wherein the first syntax element ispt_dpb_output_delay, the second syntax element ispt_dpb_output_du_delay, and the third syntax element ispt_display_elemental_periods_minus1.

C8. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a syntax element associated with a decodingpicture buffer (DPB), when included in the bitstream, is access unit(AU) specific, and wherein the syntax element specifies a number of subclock ticks to wait after removal of a last decoding unit (DU) in an AUfrom the coded picture buffer (CPB) before one or more decoded picturesof the AU are output from the DPB.

C9. The method of solution C8, wherein the syntax element is used tocompute a DPB output time.

C10. The method of solution C8, wherein the syntax element isdui_dpb_output_du_delay.

Yet another listing of solutions preferred by some embodiments isprovided next.

D1. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a flag, when included in the bitstream, isaccess unit (AU) specific, wherein the value of the flag is based onwhether an associated AU is an intra random access points (TRAP) AU or agradual decoding refresh (GDR) AU, and wherein the value of the flagspecifies (i) whether a syntax element is present in a buffering periodsupplemental enhancement information (SEI) message and (ii) whetheralternative timing information is present in a picture timing SEImessage in a current buffering period.

D2. The method of solution D1, wherein the value of the flag is equal tozero in response to the associated AU not being an IRAP AU or a GDR AU.

D3. The method of solution D1, wherein the value of the flag is inferredto be zero in response to the flag not being included in the bitstream.

D4. The method of solution D1, wherein the value of the flag being onespecifies that the syntax element is present in the buffering period SEImessage.

D5. The method of any of solutions D1 to D4, wherein the flag isbp_alt_cpb_params_present_flag and the syntax element isbp_use_alt_cpb_params_flag.

Yet another listing of solutions preferred by some embodiments isprovided next.

E1. A method of video processing, comprising performing a conversionbetween a video comprising one or more pictures and a bitstream of thevideo, wherein the bitstream conforms to a format rule, wherein theformat rule specifies that a value of a first syntax element is based ona flag that indicates whether a temporal distance between output timesof consecutive pictures of a hypothetical reference decoder (HRD) isconstrained and a variable that identifies a highest temporal sublayerto be decoded, and wherein the first syntax element specifies a numberof elemental picture period intervals that one or more decoded picturesof the current AU occupy for a display model.

E2. The method of solution E1, wherein the variable identifies a highesttemporal sublayer to be decoded.

E3. The method of solution E1 or E2, wherein the variable is Htid.

E4. The method of solution E1, wherein the flag is a second syntaxelement included in an output layer set (OLS) timing and HRD parameterssyntax structure.

E5. The method of solution E1, wherein the first syntax element isincluded in a picture timing supplemental enhancement information (SEI)message.

E6. The method of any of solutions E1 to E5, wherein the flag isfixed_pic_rate_within_cvs_flag, the first syntax element ispt_display_elemental_periods_minus1, and the variable is Htid.

The following applies to one or more of the aforementioned solutions.

O1. The method of any of the preceding solutions, wherein the conversioncomprises decoding the video from the bitstream.

O2. The method of any of the preceding solutions, wherein the conversioncomprises encoding the video into the bitstream.

O3. A method of storing a bitstream representing a video to acomputer-readable recording medium, comprising generating the bitstreamfrom the video according to a method described in any one or more of thepreceding solutions; and storing the bitstream in the computer-readablerecording medium.

O4. A video processing apparatus comprising a processor configured toimplement a method recited in any one or more of the precedingsolutions.

O5. A computer-readable medium having instructions stored thereon, theinstructions, when executed, causing a processor to implement a methodrecited in one or more of the preceding solutions.

O6. A computer readable medium that stores the bitstream generatedaccording to any one or more of the preceding solutions.

O7. A video processing apparatus for storing a bitstream, wherein thevideo processing apparatus is configured to implement a method recitedin any one or more of the preceding solutions.

Yet another listing of solutions preferred by some embodiments isprovided next.

P1. A video processing method, comprising performing a conversionbetween a video comprising one or more video pictures and a codedrepresentation of the video, wherein the coded representation conformsto a format rule, wherein the format rule permits use of a random accessdecodable leading (RADL) subpicture in a random access skipped leading(RASL) picture as a reference subpicture for predicting a collocatedRADL picture in a RADL picture associated with a same clean randomaccess picture as the RASL picture.

P2. A video processing method, comprising performing a conversionbetween a video comprising one or more video pictures and a codedrepresentation of the video, wherein the coded representation conformsto a format rule, wherein the format rule specifies that a picturetiming supplemental enhancement information message, when included inthe coded representation, is access unit specific and whereincorresponding random access skipped leading (RASL) pictures must includeRASL network abstraction layer unit type (NUTs).

P3. The method of solution P1 or P2, wherein the performing theconversion comprises parsing and decoding the coded representation togenerate the video.

P4. The method of solution P1 or P2, wherein the performing theconversion comprises encoding the video into the coded representation.

P5. A video decoding apparatus comprising a processor configured toimplement a method recited in one or more of solutions P1 to P4.

P6. A video encoding apparatus comprising a processor configured toimplement a method recited in one or more of solutions P1 to P4.

P7. A computer program product having computer code stored thereon, thecode, when executed by a processor, causes the processor to implement amethod recited in any of solutions P1 to P4.

In the present disclosure, the term “video processing” may refer tovideo encoding, video decoding, video compression or videodecompression. For example, video compression algorithms may be appliedduring conversion from pixel representation of a video to acorresponding bitstream representation or vice versa. The bitstreamrepresentation (or simply, the bitstream) of a current video block may,for example, correspond to bits that are either co-located or spread indifferent places within the bitstream, as is defined by the syntax. Forexample, a macroblock may be encoded in terms of transformed and codederror residual values and also using bits in headers and other fields inthe bitstream.

The disclosed and other solutions, examples, embodiments, modules andthe functional operations described in this disclosure can beimplemented in digital electronic circuitry, or in computer software,firmware, or hardware, including the structures disclosed in thisdisclosure and their structural equivalents, or in combinations of oneor more of them. The disclosed and other embodiments can be implementedas one or more computer program products, i.e., one or more modules ofcomputer program instructions encoded on a computer readable medium forexecution by, or to control the operation of, data processing apparatus.The computer readable medium can be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter effecting a machine-readable propagated signal, or a combinationof one or more them. The term “data processing apparatus” encompassesall apparatus, devices, and machines for processing data, including byway of example a programmable processor, a computer, or multipleprocessors or computers. The apparatus can include, in addition tohardware, code that creates an execution environment for the computerprogram in question, e.g., code that constitutes processor firmware, aprotocol stack, a database management system, an operating system, or acombination of one or more of them. A propagated signal is anartificially generated signal, e.g., a machine-generated electrical,optical, or electromagnetic signal, that is generated to encodeinformation for transmission to suitable receiver apparatus.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile in a file system. A program can be stored in a portion of a filethat holds other programs or data (e.g., one or more scripts stored in amarkup language document), in a single file dedicated to the program inquestion, or in multiple coordinated files (e.g., files that store oneor more modules, sub programs, or portions of code). A computer programcan be deployed to be executed on one computer or on multiple computersthat are located at one site or distributed across multiple sites andinterconnected by a communication network.

The processes and logic flows described in this disclosure can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., a field-programmable gate array (FPGA) or anapplication-specific integrated circuit (ASIC).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random-access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Computer readable media suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., erasable programmable read-onlymemory (EPROM), electronically erasable programmable read-only memory(EEPROM), and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto optical disks; and compact disc,read-only memory (CD-ROM) and digital versatile disc, read-only memory(DVD-ROM) disks. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

While the present disclosure contains many specifics, these should notbe construed as limitations on the scope of any subject matter or ofwhat may be claimed, but rather as descriptions of features that may bespecific to particular embodiments of the present disclosure. Certainfeatures that are described in the present disclosure in the context ofseparate embodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in the present disclosure should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in the present disclosure.

What is claimed is:
 1. A method of video processing, comprising:performing a conversion between a video comprising one or more picturesand a bitstream of the video, wherein the bitstream conforms to a formatrule, wherein the format rule specifies that a derivation of a previouspicture in a semantics associated with a first flag and in a decodingprocess for a picture order count is based on a second flag, wherein theprevious picture is a previous picture in a decoding order that has (i)a first identifier that is the same as a slice or picture headerreferring to a reference picture list syntax structure, (ii) a secondidentifier and the second flag being both equal to zero, and (iii) apicture type different from a random access skipped leading (RASL)picture and a random access decodable leading (RADL) picture, whereinthe first flag indicates whether a third flag is present in thebitstream, wherein the second flag indicates whether a current pictureis allowed to be used as a reference picture, and wherein the third flagis used to determine a first variable associated with a picture ordercount value of a long-term reference picture.
 2. The method of claim 1,wherein the first identifier is an identifier of a layer and is a syntaxelement, and the second identifier is a temporal identifier and is avariable.
 3. The method of claim 1, wherein the first flag isdelta_poc_msb_cycle_present_flag, the second flag isph_non_ref_pic_flag, and the third flag is delta_poc_msb_cycle_lt, andwherein the first identifier is nuh_layer_id and the second identifieris TemporalId.
 4. The method of claim 1, wherein the format rulespecifies that a second variable prevNonDiscardableAu, which is used todetermine a timing of a removal of a decoding unit (DU) or a decoding ofthe DU, is access unit (AU) specific, and is derived based on the secondflag.
 5. The method of claim 1, wherein the second flag equaling onespecifies that the current picture is never used as the referencepicture, and the second flag equaling zero specifies that the currentpicture is allowed to be or not to be used as the reference picture. 6.The method of claim 1, wherein the format rule specifies that abuffering period (BF) supplemental enhancement information (SEI) messageand a picture timing SEI message, when included in the bitstream, areaccess unit (AU) specific, wherein a third variable associated with thebuffering period SEI message and a fourth variable associated with thebuffering period SEI message and the picture timing SEI message arederived based on the second flag, wherein the third variable isindicative of an AU comprising (i) the second identifier of the AU thatis equal to zero and (ii) at least one picture that is not a RASLpicture or a RADL picture and for which the second flag is equal tozero, and wherein when a current AU not being a first AU in a decodingorder, the fourth variable is indicative of a previous AU in thedecoding order comprising (i) the second identifier that is equal tozero and (ii) at least one picture that is not a RASL picture or a RADLpicture and for which the second flag is equal to zero.
 7. The method ofclaim 6, wherein the third variable is notDiscardableAu, and the fourthvariable is prevNonDiscardableAu.
 8. The method of claim 1, wherein theformat rule specifies that a derivation of a fifth variable and a sixthvariable associated with a first picture and a second picture is basedon the second flag, wherein the first picture is a current picture andthe second picture is a previous picture in a decoding order that (i)comprises the second identifier that is equal to zero, (ii) comprisesthe second flag that is equal to zero, and (iii) is not a RASL pictureor a RADL picture, and wherein the fifth variable and the sixth variableare a maximum value and a minimum value, respectively, of a pictureorder count of each of the following pictures with the first identifierthat is equal to that of the first picture: (i) the first picture, (ii)the second picture, (iii) one or more short-term reference picturesreferred to by all entries in reference picture lists of the firstpicture, and (iv) each picture that has been output with a coded picturebuffer (CPB) removal time less than the CPB removal time of the firstpicture and a decoded picture buffer (DPB) output time greater than orequal to the CPB removal time of the first picture.
 9. The method ofclaim 8, wherein the fifth variable is maxPicOrderCnt and indicates themaximum value of the picture order count, and the sixth variable isminPicOrderCnt and indicates the minimum value of the picture ordercount.
 10. The method of claim 1, wherein the conversion comprisesdecoding the video from the bitstream.
 11. The method of claim 1,wherein the conversion comprises encoding the video into the bitstream.12. An apparatus for processing video data comprising a processor and anon-transitory memory with instructions thereon, wherein theinstructions upon execution by the processor, cause the processor to:perform a conversion between a video comprising one or more pictures anda bitstream of the video, wherein the bitstream conforms to a formatrule, wherein the format rule specifies that a derivation of a previouspicture in a semantics associated with a first flag and in a decodingprocess for a picture order count is based on a second flag, wherein theprevious picture is a previous picture in a decoding order that has (i)a first identifier that is the same as a slice or picture headerreferring to a reference picture list syntax structure, (ii) a secondidentifier and the second flag being both equal to zero, and (iii) apicture type different from a random access skipped leading (RASL)picture and a random access decodable leading (RADL) picture, whereinthe first flag indicates whether a third flag is present in thebitstream, wherein the second flag indicates whether a current pictureis allowed to be used as a reference picture, and wherein the third flagis used to determine a first variable associated with a picture ordercount value of a long-term reference picture.
 13. The apparatus of claim12, wherein the first identifier is an identifier of a layer and is asyntax element nuh_layer_id, and the second identifier is a temporalidentifier and is a variable TemporalId, wherein the first flag isdelta_poc_msb_cycle_present_flag, the second flag isph_non_ref_pic_flag, and wherein the second flag equaling one specifiesthat the current picture is never used as the reference picture, and thesecond flag equaling zero specifies that the current picture might ormight not be used as the reference picture.
 14. The apparatus of claim12, wherein the format rule specifies that a second variableprevNonDiscardableAu, which is used to determine a timing of a removalof a decoding unit (DU) or a decoding of the DU, is access unit (AU)specific, and is derived based on the second flag.
 15. The apparatus ofclaim 12, wherein the format rule specifies that a buffering period (BF)supplemental enhancement information (SEI) message and a picture timingSEI message, when included in the bitstream, are access unit (AU)specific, wherein a third variable associated with the buffering periodSEI message and a fourth variable associated with the buffering periodSEI message and the picture timing SEI message are derived based on thesecond flag, wherein the third variable is indicative of an AUcomprising (i) the second identifier of the AU that is equal to zero and(ii) at least one picture that is not a RASL picture or a RADL pictureand for which the second flag is equal to zero, wherein when a currentAU not being a first AU in a decoding order, the fourth variable isindicative of a previous AU in the decoding order comprising (i) thesecond identifier that is equal to zero and (ii) at least one picturethat is not a RASL picture or a RADL picture and for which the secondflag is equal to zero, and wherein the third variable isnotDiscardableAu, and the fourth variable is prevNonDiscardableAu. 16.The apparatus of claim 12, wherein the format rule specifies that aderivation of a fifth variable and a sixth variable associated with afirst picture and a second picture is based on the second flag, whereinthe first picture is a current picture and the second picture is aprevious picture in a decoding order that (i) comprises the secondidentifier that is equal to zero, (ii) comprises the second flag that isequal to zero, and (iii) is not a RASL picture or a RADL picture,wherein the fifth variable and the sixth variable are a maximum valueand a minimum value, respectively, of a picture order count of each ofthe following pictures with the first identifier that is equal to thatof the first picture: (i) the first picture, (ii) the second picture,(iii) one or more short-term reference pictures referred to by allentries in reference picture lists of the first picture, and (iv) eachpicture that has been output with a coded picture buffer (CPB) removaltime less than the CPB removal time of the first picture and a decodedpicture buffer (DPB) output time greater than or equal to the CPBremoval time of the first picture, and wherein the fifth variable ismaxPicOrderCnt and indicates the maximum value of the picture ordercount, and the sixth variable is minPicOrderCnt and indicates theminimum value of the picture order count.
 17. A non-transitorycomputer-readable storage medium storing instructions that cause aprocessor to: perform a conversion between a video comprising one ormore pictures and a bitstream of the video, wherein the bitstreamconforms to a format rule, wherein the format rule specifies that aderivation of a previous picture in a semantics associated with a firstflag and in a decoding process for a picture order count is based on asecond flag, wherein the previous picture is a previous picture in adecoding order that has (i) a first identifier that is the same as aslice or picture header referring to a reference picture list syntaxstructure, (ii) a second identifier and the second flag being both equalto zero, and (iii) a picture type different from a random access skippedleading (RASL) picture and a random access decodable leading (RADL)picture, wherein the first flag indicates whether a third flag ispresent in the bitstream, wherein the second flag indicates whether acurrent picture is allowed to be used as a reference picture, andwherein the third flag is used to determine a first variable associatedwith a picture order count value of a long-term reference picture. 18.The non-transitory computer-readable storage medium of claim 17, whereinthe format rule specifies that a second variable prevNonDiscardableAu,which is used to determine a timing of a removal of a decoding unit (DU)or a decoding of the DU, is access unit (AU) specific, and is derivedbased on the second flag, wherein the format rule specifies that abuffering period (BF) supplemental enhancement information (SEI) messageand a picture timing SEI message, when included in the bitstream, areaccess unit (AU) specific, wherein a third variable associated with thebuffering period SEI message and a fourth variable associated with thebuffering period SEI message and the picture timing SEI message arederived based on the second flag, wherein the third variable isindicative of an AU comprising (i) the second identifier of the AU thatis equal to zero and (ii) at least one picture that is not a RASLpicture or a RADL picture and for which the second flag is equal tozero, wherein when a current AU not being a first AU in a decodingorder, the fourth variable is indicative of a previous AU in thedecoding order comprising (i) the second identifier that is equal tozero and (ii) at least one picture that is not a RASL picture or a RADLpicture and for which the second flag is equal to zero, wherein thethird variable is notDiscardableAu, and the fourth variable isprevNonDiscardableAu, wherein the format rule specifies that aderivation of a fifth variable and a sixth variable associated with afirst picture and a second picture is based on the second flag, whereinthe first picture is a current picture and the second picture is aprevious picture in a decoding order that (i) comprises the secondidentifier that is equal to zero, (ii) comprises the second flag that isequal to zero, and (iii) is not a RASL picture or a RADL picture,wherein the fifth variable and the sixth variable are a maximum valueand a minimum value, respectively, of a picture order count of each ofthe following pictures with the first identifier that is equal to thatof the first picture: (i) the first picture, (ii) the second picture,(iii) one or more short-term reference pictures referred to by allentries in reference picture lists of the first picture, and (iv) eachpicture that has been output with a coded picture buffer (CPB) removaltime less than the CPB removal time of the first picture and a decodedpicture buffer (DPB) output time greater than or equal to the CPBremoval time of the first picture, and wherein the fifth variable ismaxPicOrderCnt and indicates the maximum value of the picture ordercount, and the sixth variable is minPicOrderCnt and indicates theminimum value of the picture order count.
 19. A non-transitorycomputer-readable recording medium storing a bitstream of a video whichis generated by a method performed by a video processing apparatus,wherein the method comprises: generating the bitstream of the videocomprising one or more pictures, wherein the bitstream conforms to aformat rule, wherein the format rule specifies that a derivation of aprevious picture in a semantics associated with a first flag and in adecoding process for a picture order count is based on a second flag,wherein the previous picture is a previous picture in a decoding orderthat has (i) a first identifier that is the same as a slice or pictureheader referring to a reference picture list syntax structure, (ii) asecond identifier and the second flag being both equal to zero, and(iii) a picture type different from a random access skipped leading(RASL) picture and a random access decodable leading (RADL) picture,wherein the first flag indicates whether a third flag is present in thebitstream, wherein the second flag indicates whether a current pictureis allowed to be used as a reference picture, and wherein the third flagis used to determine a first variable associated with a picture ordercount value of a long-term reference picture.
 20. The non-transitorycomputer-readable recording medium of claim 19, wherein the format rulespecifies that a second variable prevNonDiscardableAu, which is used todetermine a timing of a removal of a decoding unit (DU) or a decoding ofthe DU, is access unit (AU) specific, and is derived based on the secondflag, wherein the format rule specifies that a buffering period (BF)supplemental enhancement information (SEI) message and a picture timingSEI message, when included in the bitstream, are access unit (AU)specific, wherein a third variable associated with the buffering periodSEI message and a fourth variable associated with the buffering periodSEI message and the picture timing SEI message are derived based on thesecond flag, wherein the third variable is indicative of an AUcomprising (i) the second identifier of the AU that is equal to zero and(ii) at least one picture that is not a RASL picture or a RADL pictureand for which the second flag is equal to zero, wherein when a currentAU not being a first AU in a decoding order, the fourth variable isindicative of a previous AU in the decoding order comprising (i) thesecond identifier that is equal to zero and (ii) at least one picturethat is not a RASL picture or a RADL picture and for which the secondflag is equal to zero, wherein the third variable is notDiscardableAu,and the fourth variable is prevNonDiscardableAu, wherein the format rulespecifies that a derivation of a fifth variable and a sixth variableassociated with a first picture and a second picture is based on thesecond flag, wherein the first picture is a current picture and thesecond picture is a previous picture in a decoding order that (i)comprises the second identifier that is equal to zero, (ii) comprisesthe second flag that is equal to zero, and (iii) is not a RASL pictureor a RADL picture, wherein the fifth variable and the sixth variable area maximum value and a minimum value, respectively, of a picture ordercount of each of the following pictures with the first identifier thatis equal to that of the first picture: (i) the first picture, (ii) thesecond picture, (iii) one or more short-term reference pictures referredto by all entries in reference picture lists of the first picture, and(iv) each picture that has been output with a coded picture buffer (CPB)removal time less than the CPB removal time of the first picture and adecoded picture buffer (DPB) output time greater than or equal to theCPB removal time of the first picture, and wherein the fifth variable ismaxPicOrderCnt and indicates the maximum value of the picture ordercount, and the sixth variable is minPicOrderCnt and indicates theminimum value of the picture order count.